1. Understanding Special Relativity - 2
VCE Physics – Study Design
Time Dilation 2 – Is time dilation real?

2. If time dilation is all about “measurements” is Time Dilation “real”?

3. Is it real?
Real impacts e.g. GPS
Thought Experiment – Twin Paradox

4. clock at either end of platform one-clock on another train
Clocks and trains
one-clock on train X X A B
clock at either end of platform Y Y
one-clock on another train
total X + Y time is less than same interval measured on platform

5. Space ships and planets
one-clock on spaceship X X A
clock on each planet B Y Y
one-clock on another spaceship
total X + Y time is less than same interval measured by planets

6. Re-using the same clock
one-clock on spaceship X X A
clock on each planet B X X
same spaceship
total X time is less than same interval measured by planets

7. Twins One twin on spaceship X X One twin on planet A A B X X
Twin returns
Twin on spaceship is younger on return!

8. Objection / Paradox Motion is relative:
Look at it from reference frame of Twin on spaceship
Twin on planet ages less!

9. Twins One twin on spaceship X One twin on planet A A
Twin on planet is younger on return?

10. Resolution Situation is not symmetrical
Cannot construct one-clock / two clock in same way
Twin on spaceship changes reference frame (accelerates at turnaround)

11. CAREFUL Acceleration is NOT “central”:
if the spaceship just accelerated out and back without the long journey there would not be much ageing
ageing is approximately just the one-clock two-clock ageing
calculate difference in age with time-dilation equation (+/- “insignificant bits”)
Twin paradox “resolved” as situation is NOT symmetrical
One twin remains in same frame of reference
Other twin changes frame of reference
OK… so this requires an acceleration but can be “minimised” and is “not the cause” of the ageing (though “cause” depends on interpretation)

12. Clearing up misconceptions
SR CAN deal with accelerations and CAN deal with Twin Paradox
Even the +/- little bit… calculus
GR is NOT required to “resolve” Twin Paradox
GR is NOT SR + acceleration
GR = SR + Gravity
Nice discussion (Scientific American): How does relativity theory resolve the Twin Paradox?
Good detailed / technical discussion: Physics FAQ (Baez) – Twin Paradox

13. Actual Twin Experiment
Hafele-Keating
Atomic clocks sent around the world on airliners
Does require GR correction, gravitational field – low vs high altitude.